It's been my theory that the severe tilt toward Umbral/Light on the First had some connection to why Fae appear (not just Pixies, but Fuath, as well). Any kind of magical technobabble could be employed to serve as a link; perhaps the tendency of light-rich aether to "still" causes freed souls to sometimes crystalize into a living form, like ice around a dust particle making snow. (This theory also supports the similarities between ice aether and Umbrally-charged aether noted when they were talking about restoring the ice element to the Empty during the Eden raids.)

I also like the theory mentioned earlier in this thread that the extra density all aether on the Source has (due to many Rejoinings) makes it unlikely or impossible for a soul to reform as a Pixie, instead forming things like Ghosts (I don't think we've seen any Ghost mobs on the First, interestingly).

Those are the main notable differences between the Source and the First, structurally, so I think they're reasonable theories!